At present, mobile networks, such as Global System for Mobile communications (GSM for short) and Universal Mobile Telecommunications System (UMTS for short), adopt circuit switching techniques called as Circuit Switched (CS for short) domains, which are capable of providing basic voice services and supplementary services based on the voice services for a user. When the CS domain accesses an IP Multimedia Core Network Subsystem (IMS for short), it evolves into an access mode, wherein the services thereof are provided by the IMS in centralized way which is called as an IMS Centralized Service (ICS for short).
FIG. 1 shows an application scene of the IMS centralized service. As shown in FIG. 1, IMS User Equipment (IMS UE for short) 101, for which an IMS network provides services, accesses an IP multimedia subsystem 105 via a Packet Switched (PS for short) access network 102. Using the techniques of the IMS centralized service, CS User Equipment (CS UE for short) 103, for which the IMS network provides services, accesses an IP multimedia subsystem 105 via a CS access network 104. Thus, the IMS network can provide services for users using different access modes.
FIG. 2 is an architecture diagram of an IMS centralized control service. As shown in FIG. 2, there are included the following network elements:                a CS UE 201, which accesses an Enhanced Mobile Switching Center Server (eMSC Server for short) via CS control signaling;        an enhanced MSC Server 202, which, as an enhanced CS MSC Server, may serve as a user agent for a Session Initiation Protocol (SIP for short) to realize interworking between CS signaling and a SIP message and take the place of a user for accessing the IMS, besides achieving the functions of call control, mobility management, and an access of CS user equipment;        a Media Gateway (MGW for short) 203, which is used for interworking between a media stream over a CS and a media stream over an IP and establishing a media connection between the user equipment 201 and a remote user;        a Call Session Control Function (CSCF for short) 204, which can be classified into Interrogating CSCF (I-CSCF for short) and Serving CSCF (S-CSCF for short), wherein the I-CSCF interacts with a Home Subscriber Server (HSS for short) to request the HSS to assign the S-CSCF for a user or inquire a S-CSCF in which the user is located, wherein the S-CSCF is used for providing functions of registration, call control, etc. for a user;        an Application Server (AS for short) 205, comprising a service continuity server, a telephony service server, and so on, which can provide a user with services of service continuity, telephony services, etc.; and        a HSS/Home Location Register (HLR for short) 206, which is used for storing user's subscription data and providing support for a call or a session, wherein the HLR can be regarded as a subset of the HSS and serves for a legacy CS domain and a packet domain. In practical application, the HLR and the HSS can be integrally provided in a same physical entity, or located in different entities. When the HSS and the HLR are located in different entities, the HSS and the HLR have interfaces therebetween for realizing information interaction.        
The CS UE 201 accesses the enhanced MSC Server 202 via CS control signaling. The enhanced MSC Server 202 serves as a user agent for taking the place of the CS UE 201 to access the CSCF 204 of the IMS network and to establish a session connection with a remote user. Meanwhile, the enhanced MSC Server 202 controls the MGW 203 to accomplish switching between a media stream over a CS and a media stream over an IP and establishes a media connection between the CS UE 201 and the remote user.
It is required in the IMS centralized service to establish a binding relationship between a user identity and an actual physical contact address, viz. to establish corresponding relationships among a Private User Identity (PVI for short), a Public User Identity (PUI for short), and an actual physical contact address of an UE. A contact address can be determined from a user identity via above corresponding relationships, so as to establish a connection with the UE.
When the CS UE accesses the IMS system, the enhanced MSC Server replaces the CS UE to initiate registration to the IMS, to establish a registration binding relationship, viz. to establish corresponding relationships among the PVI and the PUI of the CS UE and the contact address of the enhanced MSC Server. In an existing registration process, said binding relationship is determined by the PVI and the PUI.
After the location of the CS UE in the enhanced MSC Server is successfully updated and is successfully registered in the IMS, said CS UE, when moving, possibly moves into the serving area of a new MSC Server (target MSC Server), wherein the target MSC Server is possibly either a common MSC Server (viz. a MSC Server without an enhanced ICS capacity), or an enhanced MSC Server.
When the target MSC Server is an enhanced MSC Server, the current registration process is as shown in FIG. 3. For conciseness, a process of upgrading the location of a CS domain and a process of an IMS registration are mainly described in the flow process of the figure, wherein no description of a security authentication process and a CS user data insertion process is made herein. As shown in FIG. 3, there are mainly included the following steps:                step S301: an UE initiates a process of upgrading a CS location, and sends a location updating request to a target MSC Server;        step S302: the target MSC Server sends the location updating request to a HSS/HLR;        step S303: the HSS/HLR accepts the location updating and returns a location upgrading acceptance response to the target MSC Server,        
In a specific operation process, standard CS access authentication and user data insertion processes are also performed during Step S302 to Step S303, and the HSS/HLR inserts an ICS indication into user data or location updating acceptance response, indicating said user signs up for the ICS service;                step S304: the target MSC Server returns to the UE a response of successfully updating location,        
Since the target MSC Server is an enhanced MSC Server, the target MSC Server would check the ICS indication in the user data or location updating acceptance response to determine whether the user is an ICS user; wherein if it is determined that the user is an ICS user, step S308 is performed to replace the UE for performing an IMS registration process;                step S305: the HSS/HLR sends a location cancelling request to a source MSC Server;        step S306: the source MSC Server returns a location cancelling response to the HSS/HLR and deletes CS user records stored locally;        step S307: the source MSC Server replaces the CS UE to perform an IMS de-registration process; wherein since the CS UE is no longer in an active state in the source MSC Server, the source MSC Server replaces the CS UE to perform an IMS de-registration process; the source MSC Server sends a de-registration request to the S-CSCF, and the S-CSCF deletes the initial registration binding relationship (viz. the corresponding relationships among a PVI, a PUI, and a contact address of the source MSC Server); and        step S308: the target MSC Server replaces the UE to initiate a registration process of the IMS centralized service wherein after the user's location in the target MSC Server is updated successfully, the target MSC Server replaces the UE to perform an IMS registration process, the target MSC Server sends a registration request to the S-CSCF of the IMS system, and the S-CSCF establishes a new registration binding relationship (the relationships among the private user identity, the public user identity, and the contact address of the target MSC Server). Thus, the IMS updates the registration binding relationship.        
From the above processes, it can be determined that the registration process initiated by the target MSC Server and the de-registration initiated by the source MSC Server are performed independently, therefore, the sequence in which the registration request sent by the target MSC Server and the de-registration request sent by the source MSC Server reach the S-CSCF can not be determined. If the registration request reaches the S-CSCF ahead of the de-registration request, the S-CSCF performs a registration process. If it is found that the user has registered (PVI-PUI-contact address of the source MSC Server), a new registration relationship is used (PVI-PUI-contact address of the target MSC Server) to replace the old registration relationship. Subsequently, the de-registration request reaches the S-CSCF, the S-CSCF matches the binding relationship using the PVI and the PUI. If finding the user has registered (PVI-PUI-contact address of the target MSC Server), the S-CSCF would deem that the newly established registration relationship (PVI-PUI-contact address of the target MSC Server) shall be de-registered. Thus, the newly established registration relationship is de-registered, which results in logical errors of services, and might cause an error of the registration binding relationship so as to affect the normal processing of the service.